“Life is Bottled Sunshine” Photosynthesis “Life is Bottled Sunshine” Wynwood Reade, Martyrdom of Man, 1872   Taiz & Zeiger: Chapters 7 & 8 Mo: Light reactions Wd: Light reactions/carbon reactions Fri: Carbon reactions Eva Farre, s150

Topics Overview of photosynthesis Key early photosynthesis experiments Photosynthesis: light reactions

Clicker question: For the last 4 billion of years has been the oxygen concentration in the atmosphere constant? A. Yes, more of less. B. It fluctuated as did the Earth temperature. C. It started rising ~3 billion years ago. D. It has decreased. E. It started rising since the origin of earth.

Oxygenic Photosynthesis Clicker question: A. CH2 B. ATP C. O2 D. NADPH E. H2O F. CO2 Light Reactions Carbon Reactions ? ? ? ? ?

Oxygenic Photosynthesis Light Reactions Carbon Reactions ATP NADPH H20 O2 CO2 CH20

Clicker question: What is the difference between the carbon in CO2 and the carbon in CH20? A. In CO2 the carbon is more oxidized than in CH2O. B. In CO2 the carbon is more reduced than in CH2O.

Photosynthesis is a redox reaction Where does the oxygen come from ? H20 + CO2 CH20 + O2 Van Niel (1920s) analyzed the photosynthesis of purple bacteria: nCO2 + 2 nH2O + light  (CH2O)n + nO2 + nH2O nCO2 + 2nH2S + light  (CH2O)n + 2nS + nH2O Hill equation (1937): H20 + A + light  ½ O2 + H2A Van Niel (1920s) inferred correctly that CO2 was not the source of the evolved O2, or alternatively said, the initial oxidation-reduction reactions did not involve CO2. In other words, the conversion of light energy to biosynthesis using CO is not obligatory. Hill equation: Robert Hill in 1937 detected that isolated chloroplasts give off oxygen in the presence of unnatural reducing agents like iron oxalate, ferricyanide or benzoquinone after exposure to light. The reaction went down in literature as the HILL-reaction. (Ziong et al., 2000, Science) “Purple bacteria are the earliest emerging photosynthetic lineage” OXYGENIC / ANOXYGENIC photosynthesis LIGHT/ DARK or CARBON-LINKED reactions

So, what happened before photosynthesis evolved? William Martin*, John Baross‡, Deborah Kelley‡ and Michael J. Russell (Nat. Rev. Microbiology, 2008) The chemistry of life is the chemistry of reduced organic compounds, and therefore all theories for the origin of life must offer testable hypotheses to account for the source of these compounds. So, what happened before photosynthesis evolved? From Martin et al., 2008: Sources of reduced carbon (remember, energy is needed) 1. Chemical reactions catalyzed by the minerals in hydrothermal (energy) vents might lead to the production of formate and acetate 2. pH gradients generated at the hydrothermal vents could be harnessed to generate chemiosmotic coupling. Others.

H20 + CO2 CH20 + O2

Topics Overview of photosynthesis Key early photosynthesis experiments Photosynthesis: light reactions

History: From air to starch Stephen Hales (1677-1761): Plants assimilated “air” Joseph Priestley (1733–1804) : plants can “purify” air Jan Ingen-Housz (1730-1799): this was dependent on light Jean Senebier (1742–1809) :”fixed air” (CO2) is essential for photosynthesis Pierre Joseph Pelletier (1788– 1842) and Joseph Bienaimé Caventou (1795–1877): chlorophyll Hugo von Mohl (1805–1872) discovered chloroplasts in plant cells Julius Robert Mayer (1814–1878): plants convert light energy into chemical energy Julius von Sachs (1832–1897 ): starch granules are product of photosynthesis Stephen Hales (1677-1761): He found a decrease in the volume of air when he grew plants in a closed environment Charles Bonnet (1720-1793) : illuminated leaves produced bubbles under water Joseph Priestley (1733–1804) : plants can “purify” air Jan Ingen-Housz (1730-1799), he found that Priestley experiment was dependent on light (1779) Jean Senebier (1742–1809) :”fixed air” (CO2) is essential for photosynthesis Pierre Joseph Pelletier (1788– 1842) and Joseph Bienaimé Caventou (1795–1877) named the green plant pigment chlorophyll Hugo von Mohl (1805–1872) dis- covered chloroplasts in plant cells Julius Robert Mayer (1814–1878) pro- posed ‘the law of conservation of energy,’ known also as the First Law of Thermodynamics. He clearly stated that ‘plants convert light energy into chem- ical energy’ during photosynthesis Julius von Sachs (1832–1897 ): starch granules are product of photosynthesis

Topics Overview of photosynthesis Key early photosynthesis experiments Photosynthesis: light reactions

Oxygenic Photosynthesis Light Reactions Carbon Reactions ATP NADPH H20 O2 CO2 CH20

Chloroplast structure

Photosynthesis Light Reactions (Simplified) e-acceptor (NADP) ADP + Pi e- Reduced e-acceptor (NADPH) ATP Stroma Thylakoid Lumen What does this scheme resemble to: respiration in mitochondria e- H+ e-Donor (H20) H+ Oxidized-e-Donor (O2)

Absorbance of light: Photochemistry There are 200-300 chlorphylls per reaction center in higher plants. Some photosynthetic bacteria have 20-30 chlorophylls Some types of algae and bacteria can have up to a few thousands per reaction center The molecular structures of antenna pigments are quite diverse.

Light absorption and emission of chlorophyll E = h (c/λ) H= Planck’s constant C= light velocity λ= wavelength c/λ = frequency

Engelmann Experiments (1843 – 1909) In essence more sophisticated modern technologies to study photosynthesis rely on the same base as Engelmann Experiments: Particular parts of the photosynthesis system absorb light of a particular wave length. When their status changes (get reduced or oxidized) their absorbance spectrum changes, which allows to follow these reactions. Spyrogyra up to ~200 um long

Photosynthetic Pigments Absorption Spectra Bacteriochlorophyll a Cholorphyll a Chlorophyll b Phycoerythrobilin (bilin pigment, are open chain tetrapyrroles found in antenna structures known as phycobilisomes that occur in cyanobacteria and red algae) B-carotene

Photosynthetic Pigments : Structure Chlorophyll and billin pigments originate from glutamate Carotenoids are tetraterpene molecules derived from eight isoprene units (isoprenes derive from Acetyl-CoA)

The Electron Transfer Process Energy what happens to this "excited" electron? The “activated” electron in the donor orbital has two choices, to transfer to another orbital of another molecule as shown here of “come down” to its original orbital. The decision is made by the different kinetics of these two options: The faster reaction occurs. The photosynthesis have evolved to optimize this reaction.

The Electron Transfer Process Clicker question: What happens to this "excited" electron? 1. Excited chlorophyll re-emits a photon and returns to its ground state (fluorescence). 2. Excited chlorophyll returns to its ground state by directly converts its excitation energy to heat? 3. Excited chlorophyll transfers its energy to another pigment. 4. Energy is used to catalyze a chemical reaction. A: 1, 3 B: 1, 4 C: all C: 1, 2 D: 2, 3, 4 The “activated” electron in the donor orbital has two choices, to transfer to another orbital of another molecule as shown here of “come down” to its original orbital. The decision is made by the different kinetics of these two options: The faster reaction occurs. The photosynthesis have evolved to optimize this reaction.

Important photosynthesis concepts Photochemical quantum yield/ efficiency: Φ = Number of photochemical products Total number of quanta absorbed (energy of a photon) Energy efficiency: = Notal number of quanta absorbed Photosynthesis products (ATP/NADPH) Quantum yield of photochemistry is very high, close to perfect 1 (little energy loss) in chloroplasts kept in dim light and the quantum yield of fluorescence is 0.05 or lower. Energy efficiency is about ~30% (modern solar panels have an efficiency of ~30%)

The Electron Transfer Process 2 H+ + ½ O2 Energy The “activated” electron in the donor orbital has two choices, to transfer to another orbital of another molecule as shown here of “come down” to its original orbital. The decision is made by the different kinetics of these two options: The faster reaction occurs. The photosynthesis have evolved to optimize this reaction.

Red light enhancement effect (~1950s) : 2 photochemical complexes (~1960) The rate of photosynthesis when red and far-red light are given together is greater than the sum of the rates when they are given apart.

Z- Scheme of photosynthesis

Photosynthesis: Light Reactions The electrons from photosystem II are transferred to Pheophytin. Pheophytin is a chloprhyll in which the central magnesium atom has been replaced by two hydrogen atoms. This gives pheophytin chemical and spectral properties that are slightly different from those of chlorophyll. Two plastoquinones PQA and PQB are bound to the reaction center and receive electrons from pheophytin in a sequential fashion. PQB2- takes then protons from the stroma to form fully reduced plastohydroquinone.The reduced plastohydroquinone (PQH2) dissociates from the reaction center complex and and entersthe hydrocarbon portion of the membrane, where it in turn transferes its electrons to the cytochrome b6f complex.

Z-Scheme: Electron transfers (redox reactions)

Photosynthesis: Light Reactions The electrons from photosystem II are transferred to Pheophytin. Pheophytin is a chloprhyll in which the central magnesium atom has been replaced by two hydrogen atoms. This gives pheophytin chemical and spectral properties that are slightly different from those of chlorophyll. Two plastoquinones PQA and PQB are bound to the reaction center and receive electrons from pheophytin in a sequential fashion. PQB2- takes then protons from the stroma to form fully reduced plastohydroquinone.The reduced plastohydroquinone (PQH2) dissociates from the reaction center complex and and entersthe hydrocarbon portion of the membrane, where it in turn transferes its electrons to the cytochrome b6f complex.

7.31 Summary of the experiment carried out by Jagendorf and coworkers: the chemiosmotic mechanism pp4e-fig-07-31-0.jpg

Topics Overview of photosynthesis Key early photosynthesis experiments Photosynthesis: light reactions

What are the main open questions about the origin of oxygenic photosynthesis? Which are the current hypothesis to explain the beginning of oxygenic photosynthesis?

Next lecture: -Genetics and Evolution of Photosynthetic Systems -Regulation of the Photosynthetic Machinery -Carbon reactions: Calvin-Benson cycle

Monday’s Topics Overview of photosynthesis Key early photosynthesis experiments Photosynthesis: light reactions